The size and distribution of lipoprotein particles in the intestinal epithelium were determined before and after the start of suckling by infant rats. Prior to suckling very low density lipoprotein particles were within the smooth endoplasmic reticulum (SER), rough endoplasmic reticulum (RER), Golgi apparatus and interepithelial cell spaces of duodenal villus tip cells. Chylomicrons were produced in the SER, RER and Golgi apparatus of duodenal epithelial cells after the commencement of suckling.
Ileal epithelial cells did not contain particles of VLDL dimensions prior to suckling. Ileal cells of rats suckled for 24 hours after birth had VLDL particles in the SER, RER and Golgi apparatus above and at the sides of the large supranuclear vacuole in each cell. Ileal cells at 10 and 15 days of age had fewer and smaller particles. These particles were also present in the interepithelial cell spaces and lacteals.
It is suggested that VLDL particles are produced by duodenal cells prior to suckling when only endogenous lipid is available. The ingestion of exogenous lipids in milk is associated with the production of chylomicrons by duodenal epithelial cells and VLDL particles by ileal epithelial cells. VLDL particles within ileal cells become smaller and less numerous as suckling continues, possibly due to increased pancreatic lipase. This is interpreted as evidence that chylomicrons and intestinal VLDL particles are related.